1. Field of the Invention
This invention relates to electronic circuits for controlling the power to brushless direct current motors, and more particularly to actively controlling the slew rate of the power transistors which supply the current to the stator windings of a brushless direct current motor when the stator coils are commutated.
2. Description of the Relevant Art
The present invention pertains to polyphase direct current (dc) motors, in general, and particularly to three phase dc motors which may be of the brushless, sensorless type which are used for rotating data media, such as found in computer related applications, including hard disk drives, CD ROM drives, floppy disks, and the like. In computer applications, three phase brushless, sensorless dc motors are becoming more popular, due to their reliability, low weight, and accuracy.
FIG. 1 illustrates the typical architecture of a brushless polyphase direct current motor as described in detail in U.S. Pat. Nos. 5,172,036 and 5,204,594 which are fully incorporated into this specification by reference. Specifically, FIG. 1 shows that the motor 12 consists of a stator 16 and a rotor 14. The appropriate phase of the motor is determined by Hall effect sensors 103 or by monitoring the back electromotive force (BEMF) on the floating coil. Thus, the commutator circuit 20 determines the appropriate driver circuit 10 to enable.
FIG. 2 shows a general typical schematic of the output stage of driver circuit 22 of FIG. 1. The method and apparatus for operating a polyphase motor direct current motor is more fully explained in U.S. Pat. No. 5,221,881 and is fully incorporated into this specification by reference.
Motors of this type can typically be thought of as having a stator with three coils connected in a "Y" (wye) configuration, although actually, a larger number of stator coils are usually employed with multiple motor poles. FIG. 1 shows a stator in a wye configuration as element 16. Typically, in such applications, eight pole motors are used having twelve stator windings and four N-S magnetic sets on the rotor resulting in four electrical cycles per revolution of the rotor. In bipolar operation, the coils are energized in a sequences such that a current path is established through two coils of the wye with the third coil left floating. The sequences are arranged so that as the current paths are changed, or commutated, one of the coils of the current path is switched to float, and the previously floating coil is switched into the current path. Moreover, the sequence is defined such that when the floating coil is switched into the current path, current will flow in the same direction in the coil which was included in the prior current path. Therefore, six commutation sequences are defined for each electrical cycle in a three phase motor as given below in Table A.
TABLE A ______________________________________ CURRENT FLOWS FLOATING PHASE FROM: TO: COIL: ______________________________________ 1 A B C 2 A C B 3 B C A 4 B A C 5 C A B 6 C B A ______________________________________
Another common mode of operation is the unipolar mode where one stator coil winding is energized at a time. This is accomplished by either grounding the center tap of the stator windings while sequentially energizing each stator winding, or by tying the center tap to the voltage supply and sequentially grounding the other end of each stator winding. In unipolar operation, it may be desirable to allow the voltage on the high side drivers to go several volts above Vcc to detect zero crossings, or determine rotor position, or the like.
In the either unipolar or bipolar operation of the motor, large voltage spikes are generated when the phases are commutated since the operation requires that the motor current be redirected from one stator winding to another. For example, referring now to FIG. 2, in phase 1 of Table A above, transistor 44 and transistor 45' of FIG. 2 are on which allows current to flow from the voltage source through transistor 44, stator winding A, stator winding B, and transistor 45' During commutation from phase 1 to phase 2, transistor 45' is turned off while transistor 45" is turned on. This causes a voltage spike in the stator winding B due to the collapsing electromagnetic field which was created by the current flowing in stator winding B. The voltage spike on the stator winding is a function of the rate (di/dt) at which the stator winding current is turned off and can be described as dV=-L(di/dt) where dV is the differential voltage, L is the inductance of the stator winding, and (di/dt) is the rate at which current is changing as a function of time. Therefore, the quicker the current is turned off and the larger the inductance of the stator coil, the larger the voltage spike.
In the past, this voltage spike was clamped using a diode such as the diodes 47, 47', 47" and 48, 48', 48" in FIG. 2. To illustrate how the diodes work in the circuit, assume again that the circuit is in phase 1 and will commutate to phase 2 of Table A. While in phase 1, current flows through transistor 44, stator winding A, stator coil B, transistor 45', and through sense resistor 49 to ground. Commutation occurs by turning 45' off while turning 45" on, the result of which is to redirect the current from stator winding B to stator winding C. Since the current in stator winding B has gone from some significant value to zero in a relatively short amount of time, an inductive voltage spike is generated. Therefore, the voltage potential at node "out b" is driven above the source voltage by the voltage spike. As the voltage potential at node "out b" rises above the turn-on threshold of the diode, diode 47' turns on and clamps the voltage spike to the voltage source. The turn-on voltage of a diodes is typically around 700 millivolts. Diodes 47 and 47" serve the same functions for stator windings A and C, respectively. Similarly, diodes 48, 48', and 48" clamp the voltage spikes which are created when stator windings A, B, and C are turned off after being turned on by transistors 44, 44', and 44", respectively.
These clamping diodes 47, 47', 47", 48, 48' and 48" have typically been either external or internal diodes. However, the diodes can be replaced by using a synchronous clamping technique as described in copending U.S. patent application Ser. No. 8/250,027 filed on May 27, 1994 which is wholly incorporated into this specification by reference. The aforementioned patent application teaches the technique of turning on the appropriate stator winding driver transistor when a voltage spike due to commutation is sensed.
The problem addressed by this invention occurs when stator winding driver transistors are used to clamp the recirculation voltage spike. It is well know in the art that the recirculation voltage spike due to commutation is a function of the slew rate of the driver transistors. It is also known in the art that the slew rate of the driver transistors are programmable to allow system integrators the capability to minimize EMI noise and system noise. In this environment, it has been observed that it is possible for system integrators to program slew rates which are so fast that the voltage spike could damage the integrated circuit before the clamp has time to turn on.
Therefore, it is an object of the invention to allow for fast slew rates without subjecting the integrated circuit to excessively high voltages.
It is further an object of this invention to provide reliable synchronous clamping without limiting the slew rate during commutation.
It is further an object of the invention to provide a programmable slew rate for commutation and a fixed slew rate for clamping.